Many portions of engines, such as gas turbine engines, become extremely hot during service. Some components are contacted by hot combustion gases whose temperatures exceed the melting points of the materials of construction of the components. A number of techniques are used to allow the components to operate under such conditions. In one, the surface of the material is insulated by a protective thermal barrier coating.
In another technique, the component is actively cooled by a flow of cooling air that passes over its surface to allow it to continue functioning. High pressure turbine blades, for example, are typically hollow and have surface openings therethrough. Compressed cool air is passed into the hollow interior of the turbine blades and exits through the surface openings. The air streams along the surfaces of the turbine blades to both cool the surfaces and provide a cool-air film layer between the hot combustion gas and the metal of the turbine blade. In a related approach, a jet of cool air may be directed against the surface of an article to be cooled.
Transpiration cooling has also been used. The article to be cooled is made to be porous. Compressed cooling air is forced through the porous article to remove heat. Transpiration cooling has an advantage that the cooler air remains in contact with the material of the article for a relatively long period of time so that a significant amount of heat may be transferred into the air and thence removed from the article.
A number of techniques are known for fabricating an article having a porous structure. The techniques are relatively cumbersome and time-consuming to practice, so that the cost of the article is high. Consequently, they have not found widespread use in gas turbine and other applications. If the advantages of transpiration cooling are to be realized in practice, there is a need for an improved material and method for its preparation.
The present invention provides a structure including a porous article that is transpiration cooled. It is suitable for applications in gas turbine and other types of engines. The porous article is prepared much more economically than prior types of porous articles suitable for such uses.
A structure comprises a cooled article comprising an open-cell solid foam of cell walls having a porous interconnected intracellular volume therebetween. The cell walls are formed of a material selected from the group consisting of a metal and a ceramic. A source of a pressurized gas is in communication with a source region of the cooled article. The source of the pressurized gas may comprise a gas plenum in gaseous communication with the source region, and a compressor having a compressed gas output in gaseous communication with the gas plenum.
The structure typically is a portion of an engine, such as a gas-turbine engine. In a gas turbine engine, components such as a gas-turbine blade, a gas-turbine vane, or a gas-turbine stationary shroud may benefit from this approach.
The cell walls may be a ceramic or a metal such as a nickel-base metallic alloy. In some embodiments, at least some of the cell walls are a ceramic and some of the cell walls are a metal. The ceramic material comprises a base ceramic such as aluminum oxide. The cooled article comprises at least about 60 volume percent of ceramic, most preferably from about 60 to about 80 percent by volume of ceramic.
A method of preparing a structure which includes an open-cell solid foam article comprises the steps of providing a piece of a sacrificial ceramic having the shape of a cooled article, and contacting the piece of the sacrificial ceramic with a reactive metal which reacts with the sacrificial ceramic to form an open-celled ceramic foam article.
The article comprises ceramic cell walls of an oxidized ceramic of the reactive metal, and a porous interconnected intracellular volume therebetween filled with an intracellular metal. At least a portion of one of the ceramic cell walls and the intracellular metal of the article is removed to form a transpiration volume. A source of a pressurized gas is placed in gaseous communication with a source region of the transpiration volume of the cooled article.